This kind of drive unit of a vehicle of is disclosed in Japanese Patent Laid-Open No. 2003-127681. Objectives of the invention disclosed in Japanese Patent Laid-Open No. 2003-127681 is to suppress an increase in size of an electric motor, and to obtain driving force in accordance with a various vehicle speeds. In the hybrid-vehicle drive system taught by Japanese Patent Laid-Open No. 2003-127681, an internal combustion engine is connected with a carrier of a planetary gear mechanism, a first electric motor/generator is connected with a sun gear of the planetary gear mechanism, and a member of an input side of a transmission is connected with a ring gear. The transmission is a geared transmission such as an automatic transmission. A member of an output side of the transmission is connected with a propeller shaft, and a second electric motor/generator is connected with the propeller shaft. Thus, according to this hybrid-vehicle drive system, the planetary gear mechanism constitutes a distribution mechanism for distributing power of the engine to the first electric motor/generator and to the transmission, and a revolution frequency of the ring gear, i.e., an input revolution of the transmission connected with the ring gear is varied continuously by changing a revolution frequency of the first electric motor/generator. That is, the planetary gear mechanism and the first electric motor/generator function as a continuously variable transmission. Therefore, a total speed change ratio of the hybrid-vehicle drive system is governed by a speed change ratio of the planetary gear mechanism functioning as a continuously variable transmission, and a speed change ratio of the transmission placed on the output side of the planetary gear mechanism.
A main objective of a drive unit of a vehicle is to improve fuel economy entirely by carrying out a regeneration of energy, in addition to controlling a speed change ratio. However, not only a good fuel economy and low emission, but also an excellent traveling performance such as a driving performance and a braking ability, and an excellent control response are required for a vehicle. The hybrid-vehicle drive system disclosed in Japanese Patent Laid-Open No. 2003-127681 is capable of improving the fuel economy of the vehicle. However, in the hybrid-vehicle drive system disclosed in Japanese Patent Laid-Open No. 2003-127681, an entire speed change ratio thereof is set by both speed change ratio of the continuously variable transmission unit and a gear stage of the geared transmission. Therefore, in case of increasing the speed change ratio to use an engine brake (or a power source braking), a speed change operation has to be carried out with a switching operation of an engagement device of the geared transmission such as a clutch. For this purpose, an intricate hydraulic control has to be carried out and this causes a delay in the speed change operation. Especially, in case of carrying out a downshifting by a manual operation, such delay in the speed change may bring discomfort.